This will create 7z files that are based on the last modified date of the file and it will create an archive for each date and add files to them.

archive.sh

#!/bin/bash
ARCHIVE_FOLDER=/path/to/archive/location
for i in `find -type f -mtime +14`
do
  mod=$(date -d"$(stat --printf='%y\n' $i)" +%Y%m%d)
  7za a $ARCHIVE_FOLDER/$mod.7z $i
  echo "Processed: " $i
  rm $i
done